Microwave frequency signal processing is a common aspect of many modern electronic and/or optical systems. One such system is Doppler radar. If light of a single frequency is incident on a reflecting object that has some component of motion along the light's direction of travel, the frequency of the light will be changed by an amount related to the speed of the object. This is called the Doppler effect. The velocity of the object may be determined if the frequency of the reflected light is compared with that of the incident light. This comparison will be made automatically if the return light and the reference light fall on the receiver. The photocurrent produced will contain components related to the frequency difference that describes the velocity of the reflector, provided that the polarizations are parallel. If two reflectors with different velocities are present, two RF tones will be present in the stimulus. If the receiver is linear each reflector will be represented by a unique RF tone. Otherwise, other tones will be present. This is an important problem if these tones (distortion) are very close to those actually generated by the reflectors. Such tones will be created by 3rd order harmonic distortion. They are difficult to distinguish from those originating from actual reflectors and cannot be filtered out of the RF response since they are mixed in with genuine signal. This is illustrative of one motive to characterize the linearity of an optical receiver. A receiver's 3rd order distortion is often characterized in the laboratory using tones produced synthetically. Therefore, a need exists for an effective method and apparatus for signal generation that can be useful for the testing of photodiodes for intermodulation product distortion.
Two RF tones similar to those originating from moving reflectors may be produced from three optical frequencies if one of the three pairings can be eliminated. This is important since the third tone is not independent of the other two and will create problems in the Two Tone test of a receiver.
Further, many systems applications, such as in microwave signal generation and local oscillator distribution systems for space based radars and antenna systems, require the generation and delivery of very "clean" microwave signals, i.e., signals without unwanted harmonic mixing products and of narrow line width that is achieved via the phase locked loop.
The present invention provides a method and apparatus to meet such needs.